1. Field of the Invention
This invention relates to asphalt concretes. More particularly, it relates to sulfur-extended asphalt mixtures and sand-asphalt-sulfur systems used for paving and the like.
2. Description of the Related Art
The presence of sulfur in asphaltic mixtures provides improved compaction and strength benefits which improve the durability and performance of the composition as a paving material. Research into the use of sulfur in asphalt paving materials has resulted in the development of two distinct technologies whose basic difference lies in the primary role sulfur plays in the mixture. The first development was Sand-Asphalt-Sulfur (SAS) which involves the use of sulfur as a structuring agent with poorly graded sands to produce a quality asphaltic paving material (i.e., the role of the sulfur is that of aggregate). Sulfur-Extended-Asphalt (SEA) is a later development in which sulfur is utilized as in integral part of the binder to effect a partial replacement or extension of the asphalt cement in conventional asphaltic pavement materials. These two distinct uses of sulfur in asphaltic mixtures are discussed at length in a review article by W. J. Rennie in "Sulphur Asphalts", New Uses for Sulphur--SUDIC Technology Series No. 2, 2nd ed., 1979.
The preparation of SAS materials involves a two cycle process. The operation begins with all three ingredients--aggregate, asphalt, and sulfur--preheated to a temperature above the melting point of sulfur (240.degree. F.) and below 300.degree. F. The upper limit is the temperature above which sulfur begins to undergo an abrupt and very large increase in viscosity which could adversely affect mix workability. Although acceptable mixes have been prepared at temperatures as high as 380.degree. F., 300.degree. F. is considered to be the maximum mix processing temperature for safety reasons since above this temperature, sulfur can emit toxic fumes.
In the first cycle of the preparation of SAS mixtures, aggregate and asphalt are mixed to coat the particles with asphalt. Liquid sulfur is then added and mixed with the asphalt and aggregate until the three ingredients are dispersed throughout the mix. Upon cooling, the sulfur which has not dissolved in the asphalt solidifies within the voids of the mixture thereby creating a mechanical interlock from which the material derives its strength. By acting as a conforming filler, the crystallized sulfur induces such a high degree of mechanical stability to the mix that high quality paving materials can be achieved using poorly graded aggregates such as single-sized sands. Upon consideration of a number of criteria, it has been concluded that an optimum SAS system would have sulfur and asphalt contents between 12 and 14 percent and 5 and 7 percent, respectively. [D. Saylak and W. E. Conger, "A Review of the State of the Art of Sulfur Asphalt Paving Technology," Sulfur: New Sources and Uses, American Chemical Society Symposium Series, No. 183, pp. 155-193 (1982)].
Although commercial processes for modifying properties of asphalts with sulfur have been in existence for more than a century, the current developments in the use of Sulfur Extended Asphalt (SEA) binders originated with the work of Bencowitz and Boe in 1938 ["Effect of Sulphur Upon Some of the Properties of Asphalts", Proceedings of the American Society of Testing Materials, 39 (II), p. 539]. Using a wide variety of types and sources of asphalts, they produced stable blends containing 25 percent sulfur. Blends with as much as 40% sulfur were achieved with some asphalts.
These early studies together with more recent investigations have established some conclusions regarding the effect of sulfur on the properties of SEA binders and the hot-mix concretes in which they are used. Depending upon the temperature and the amount present, sulfur will go into solution with the asphalt, chemically react with the asphalt to form aromatic polysulfides, and form a solid-liquid dispersion with the binder. At binder ratios up to 20:80 (sulfur-to-asphalt), most of the sulfur will be dissolved by the asphalt at blending temperatures between 240.degree. and 300.degree. F. As more sulfur is added, that which does not dissolve in the asphalt will exist in the dispersed phase. Because the specific gravity of sulfur is about twice that of asphalt, the solid sulfur particles will settle out unless the blend is continuously agitated. The rate of settling is dependent on the viscosity-temperature properties of the asphalt.
The engineering properties of mixes prepared with SEA binders vary with sulfur content while maintaining their load bearing characteristics over a wide range of pavement temperatures. However, with sulfur contents in excess of 35 volume percent (52 weight percent), the mixes become sensitive to compaction temperatures below 240.degree. F. At this point the sulfur content becomes significant enough that a mechanical interlocking process similar to that encountered with SAS systems becomes a factor. This is considered the maximum permissible substitution level for normal compaction conditions. At levels below 15 volume percent (26 weight percent), properties are similar to conventional asphalt mixtures.
A sulfur-extended asphalt blend requires a fine, uniform dispersion of the liquid sulfur in asphalt. The basic difference between the techniques for SEA binder preparation used by Bencowitz et al. and those in current use is the emphasis of the latter on high shear rate mixing to generate the desirable sulfur particle size (3 to 10 microns). The size range is consistent with that required to maximize solubility of the sulfur in the asphalt and to enhance long term stability. The SEA binders produced by Bencowitz were agitated at 325 rpm for 2 hours, whereas the latest methodology relies on colloid mills, emulsifiers, turbines, homogenizers, in-line mixers, or combinations of the above. In all of the above methods the sulfur and asphalt are preblended prior to entering the mix plant.
Other investigators have determined that sufficient shearing action can be produced by the interaction between the aggregate and binder during mixing. This method, referred to as direct substitution, requires that the sulfur and asphalt be stored and metered separately into either the weigh buckets or directly into the mixing unit (pugmill or drum mixer). The direct mixing method requires no special equipment in the field for SEA binder preparation. On the other hand a number of uniquely designed processing units have been developed for preblending sulfur and asphalt.
Since sulfur has about twice the unit weight of asphalt, a given weight of sulfur occupies 1/2 the volume of an equal weight of asphalt. Early mix design procedures evolved on the basis of an equal volume substitution of sulfur for asphalt (i.e., a S:A substitution weight ratio of 2 to 1). However, construction experience in the United States and Canada indicates that, due to the improved aggregate coating capability provided by the lower viscosity of SEA binders, substitution weight ratios as low as 1.4 to 1 may be feasible. An added benefit is also realized from the structuring effect provided by the undissolved sulfur particles in high-sulfur binders. The Bureau of Mines has published a guideline manual which provides detailed treatment of mix design and construction procedures for the preparation of SEA binders. [W. C. McBee et al., "State-of-the-Art Guidelines for Design, Quality control and Construction of Sulfur-Extended Asphalt (SEA) Pavements"]. This manual provides a formula for establishing the equivalent SEA binder content to replace, on an equal volume basis, the asphalt in a conventional asphaltic concrete mix design.
Throughout its development, the use of sulfur in highway paving mixtures has raised questions regarding the pollutants generated, their environmental impact, and worker safety considerations associated with mix preparation and placement. Evolved gas analyses have been incorporated into a number of field trials in Canada and the United States. Except for two instances when paver screed temperatures exceeded 320.degree. F., all emissions were found to be below the maximum allowable. In general, it has been concluded that as long as hot sulfur paving mixtures do not exceed 300.degree. F., the emission levels of all sulfur-containing species (H.sub.2 S, SO.sub.2, SO.sub.3 and organo-sulfur compounds) will be below their respective Maximum Allowable Concentrations.
Whether SEA mixes are prepared by the preblending method or the direct substitution method, supplies of both molten sulfur and molten asphalt are required. This results in significant energy demands especially when supplies of these two materials must be maintained at the ready. There are also obvious safety hazards attendant in storing and handling such molten substances. As mentioned above, temperature control is particularly critical in the case of molten sulfur inasmuch as toxic fumes may be generated if the temperature is allowed to go too high.
A granular asphalt is described in U.S. Pat. No. 3,958,067 to Takase et al. The granules are in the shape of a polyhedron having at least one acute angle and are less than about 10 mm in size. The granules are produced in the apparatus described in U.S. Pat. No. 3,758,035. It is said that the claimed granular asphalt can be melted rapidly and easily without causing thermal decomposition, denaturing, or air pollution.
A method for producing coated bitumen pellets is described in U.S. Pat. No. 3,026,568 to Moar. The purpose of achieving the granular form is to permit the bitumen to be handled in the manner of a granulated material. It is said that in converting bitumen, whether asphalt or coal tar, into the form of finely divided pellets, granules, or droplets, it is an essential requirement to coat the pellets with an appropriate powdered mineral so as to prevent the pellets from adhering to one another. Suitable coating materials are said to be several finely ground minerals, among which are limestone, Portland cement, clay, mineral flour, and diatomaceous earth. Molten asphalt is sprayed from nozzles downwardly into an upwardly directed stream of air carrying the powdered coating material. This is said to atomize the bitumen into fine pellets and initially coat and partially cool the same to a semi-molten state. The initially coated pellets are then passed through a second zone of swirling air carrying the coating material to acquire additional coating.
U.S. Pat. No. 3,001,228 to Nack relates to production of coated solid pellets of fusible materials. Asphalt is among the suitable fusible materials listed. The process comprises forming molten droplets of the fusible material and introducing the molten droplets into a fluidized bed of finely divided coating solids. Included among the suitable finely divided coating solids are clays, natural and synthetic resins, limestone, fertilizer materials, talc, diatomaceous earth, zein, and calcium carbonate. The product of the claimed process consists of substantially spherical, coated droplets of the fusible material having a coating of discrete solid particles of finely divided coating solids adhering thereto.